Conventional, hollow fiber permeability apparatus are extensively used in the medical field, for example in hemodialysis. Examples of hollow fiber permeability apparatus used in hemodialysis are as follows: U.S. Pat. No. 4,306,972, Dialysis Apparatus, to Denti, et al.; U.S. Pat. No. 4,289,623, Hollow Fiber Dialysis, to Lee; U.S. Pat. No. 4,219,426, Dialysis Device, to Spekle, et al.; U.S. Pat. No. 4,212,744, Haemodialyzer Apparatus, to Oota; U.S. Pat. No. 4,202,776, Hollow-Fiber Permeability Apparatus, to Joh; U.S. Pat. No. 4,187,180, Hollow-Fiber Permeability Apparatus, to Joh; U.S. Pat. No. 4,201,673, Apparatus for Dialysis of Solution, to Kanno, et al.; U.S Pat. No. 4,031,012, Separatory Apparatus, to Gics; and U.S. Pat. No. 3,708,071, Hollow Fiber Membrane Device and Method of Fabricating Same, to Crowley. These hollow fiber apparatus have unidirectional blood flow through the hollow fibers from inlet to outlet. Microsolutes and water are passed through the hollow fiber membrane. Differences in the concentration of ions on each side of the membrane allow the desired ions to be drawn from the blood.
Hollow fiber membrane technology is also used in apparatus commonly known as artificial lungs. In artificial lungs, oxygen and carbon dioxide are exchanged with each other to increase blood oxygen content. Reverse osmosis apparatus also employ the technology of permeable hollow fibers. These apparatus are used in purification or desalination of water where the membrane retains virtually all ions and passes water. Examples of hollow fibers particularly adapted for use in reverse osmosis are found in the following references: U.S. Pat. No. 4,084,036, Asymmetric Hollow Acrylic Fibers, to Leonard; U.S. Pat. No. 3,953,334, Fluid Fractionating Apparatus, to Brun, et al.; and U.S. Pat. No. 3,930,105, Hollow Fibres, to Cristen, et al.
Porous, hollow fibers are used in a wide variety of permeability and filter applications because of a basic advantage over flat membranes. Available surface area is increased by choosing porous, hollow fibers thereby reducing space requirements for permeability apparatus.
Hollow fiber filters have been constructed with one end of a hollow fiber bundle closed. During filtration using a sealed end filter, fluid flows into the unsealed ends and is filtered across the membrane of the porous, hollow fibers. Alternatively, filtrate can flow across the membrane of the porous, hollow fibers and exit through the single open end area. Fluid filtered at the end farthest from the main flow stream has to flow through the entire length of fiber in either alternative. This reduces the efficiency of the filter. A large percentage of the pressure differential is used to move fluid within the hollow fibers rather than across the membrane of the hollow fibers for filtration. Consequently, pressure differentials are high and flow rates are low.
A filter can also be constructed by bending the open ends of the bundle of fibers back on themselves in a generally "U"-shaped configuration, and feeding into both ends. For example, see U.S. Pat. No. 4,075,100, Dialysis Unit and Dialysis Apparatus Employing the Dialysis Unit, to Furuta, et al. and U.S. Pat. No. 4,025,436, Liquid Treatment Apparatus, to Tsuda, et al. Other difficulties must be addressed where generally "U"-shaped configuration filters are used. When the hollow fibers are bent in a "U" shape, hollow fibers close to the smaller, inner radius of the "U" can be pinched shut. Hollow fibers close to the larger, outer radius are under tension and can be flattened shut. Compromising the arc of the "U" bend to avoid these problems also compromises the overall size of the filter. Furthermore, a shorter and stouter "U"-shaped device is more cumbersome to pot and its transverse dimension is inconveniently large.
It is desirable to provide a microfilter where the pressure differential across the filter membrane can be lowered and where the filtrate flow rate is increased, thereby increasing the overall efficiency of the microfilter. Furthermore, the disadvantages encountered in the manufacture of a "U"-shaped filter, namely pinched shut and flattened shut hollow fibers, and potting difficulties, would desirably be avoided without compromising the size of the filter or the ease of manufacture. Important, too, is the adaptability of a more efficient filter for use in presently existing industrial hardware. It would also be desirable to provide a microfilter where small leaks in the filter membrane can be repaired without discarding the entire filter.